1. Field of the Invention
The present invention relates, in general, to a method and system to be utilized in data processing systems. In particular, the present invention relates to a method and system to be utilized in data processing systems wherein, for non-limiting example, a memory controller is utilized.
2. Description of the Related Art
Data processing systems are systems that manipulate, process, and store data and are notorious within the art. Personal computer systems, and their associated subsystems, constitute well known species of data processing systems. Personal computer systems in general and IBM compatible personal computer systems in particular have attained widespread use for providing computer power to many segments of today""s modern society. A personal computer system can usually be defined as a desk top, floor standing, or portable microcomputer that includes a system unit including but not limited to a system processor and associated volatile and non-volatile memory, a display device, a keyboard, one or more diskette drives, one or more fixed disk storage devices, and one or more data buses for communications between devices. One of the distinguishing characteristics of these systems is the use of a system board to electrically connect these components together. These personal computer systems are information handling systems which are designed primarily to give independent computing power to a single user (or a relatively small group of users in the case of personal computers which serve as computer server systems) and are inexpensively priced for purchase by individuals or small businesses.
A computer system or data-processing system typically includes a system bus. Attached to the system bus are various devices that may communicate locally with each other over the system bus. For example, a typical computer system includes a system bus to which a central processing unit (CPU) is attached and over which the CPU communicates directly with a system memory that is also attached to the system bus.
In addition, the computer system may include a peripheral bus for connecting certain highly integrated peripheral components to the CPU. One such peripheral bus is known as the Peripheral Component Interconnect (PCI) bus. Under the PCI bus standard, peripheral components can directly connect to a PCI bus without the need for glue logic. Thus, PCI is designed to provide a bus standard on which high-performance peripheral devices, such as graphics devices and hard disk drives, can be coupled to the CPU, thereby permitting these high-performance peripheral devices to avoid the general access latency and the band-width constraints that would have occurred if these peripheral devices were connected to a low speed peripheral bus. Details on the PCI local bus standard can be obtained under the PCI Bus Specification, Revision 2.1, from the PCI Special Interest Group, which is hereby incorporated by reference in its entirety.
Two relatively high-bandwidth types of traffic that are communicated to and from system memory are 1394 device traffic and networking traffic. The 1394 device traffic originates within a high speed serial device which communicates with the system through and over a Southbridge. The networking traffic originates within a network card which is reading network traffic information, regarding one or more networks of which the data processing system is a part, from a network buffer.
Relatively recently, techniques for rendering three-dimensional (3D) continuous-animation graphics have been implemented within PCs which have exposed limitations in the originally high performance of the PCI bus. The AGP interface standard has been developed to both (1) reduce the load on the PCI bus systems, and (2) extend the capabilities of systems to include the ability to provide 3D continuous-animation graphics with a level of quality previously found only on high-end computer workstations. The AGP interface standard adds an additional bus to data processing systems: the AGP Interconnect. The AGP interface standard is defined by the following document: Intel Corporation, Accelerated Graphics Port Interface Specification, Revision 1.0 Jul. 31, 1996).
The AGP interface standard reduces the load on PCI bus systems and extends the capabilities of systems to include the ability to provide 3D continuous-animation graphics via a rather indirect process. Under the AGP interface standard, a CPU independently processes the geometric and texturing data (geometric and texturing data are data necessary to properly define an object to be displayed) associated with each object to be displayed in a scene. Subsequent to processing the geometric and texturing data, the CPU writes the geometric and texturing data back into system memory. Thereafter, the CPU informs a graphics processor that the information is ready, and the graphics processor retrieves the information from the system memory.
In current industry architectures, each preceding discussed bus (e.g., the system bus, the AGP interconnect, and the PCI bus) independently articulate with the system memory through a device known as the Northbridge. The various communications with, or accesses of, system memory are generally controlled by a device within the Northbridge known as a xe2x80x9cmemory controller.xe2x80x9d
A memory controller controls system memory which is typically a collection of Direct Random Access Memory chips (DRAMs). The computer system memory, composed of DRAMs, can store data, but there is conventionally no intelligence in the system memory. The intelligence concerning how data is going to be stored, where the data is going to be stored, how the data is going to be read or written, etc., is provided by the xe2x80x9cmemory controller.xe2x80x9d
The memory controller controls access to system memory, which as has been noted is typically composed of DRAMs. A DRAM can be thought of as a collection of cells, or storage locations, wherein data is stored. For simplicity it will be assumed here that each cell stores a byte, but those skilled in the art will recognize that other storage sizes are possible.
When a memory access, such as a read cycle, is engaged in, the memory controller is given an address by another device, such as a graphics controller. That address needs to correctly specify one of the cells where data is actually stored. Ordinarily, cells within DRAMs are arranged in row and column format (i.e., the cells are arranged like a matrix).
Consequently, an address, which for sake of illustration will be assumed to be 16 bits long, customarily is conceived of as being composed of two parts: a first 8-bit portion of the address which is associated with a row address, and a second 8-bit portion which is associated with a column address (again, the bit lengths are hypothetical and merely utilized here for illustrative purposes). This fragmentation of the address into row and column portions allows the address to correctly specify a storage location, or cell, by its row and column.
Conventionally, a DRAM has at least two buses, or at least hypothetically what can be treated as two buses: a data bus, and an address bus. To minimize DRAM hardware, it is customary that the address bus be only half as wide as the address bits required to uniquely identify a data cell. The foregoing is done in order to minimize the number of pins on the DRAM, which those skilled in the art will recognize is a major constraint or limiting factor on how small one can make a DRAM chip. Due to this limitation on the width of the address bus, memory access is typically achieved by first placing the row portion of the address on the address bus, which will select the appropriate row, and second, a short time later, placing the column portion of the address on the address bus, which will select the appropriate column. This then correctly specifies the row and column location of the storage location that is desired. At some time after the row and column information have both been specified, the data from the memory location specified by the row and column address appears on the DRAM data bus.
From the foregoing, it can be seen that in order to make a single memory read access there are three phases: a row address phase, a column address phase, and a data retrieval phase. In the past, it was noticed that typical programs tend to operate sequentially, so if there is a memory address accessed, it is likely that the next memory address accessed will be the very next cell, which means that the column address is likely to change, while the row address is not likely to change. Consequently, typical DRAMs are structured such that once the row address has been driven, thereafter the DRAM responds to new addresses on the address bus as if those addresses are column indicators, and thus will use such addresses as column addresses within a current row until the DRAM is notified that a new row address will be appearing on the address bus. DRAM devices using this scheme (driving the row once and then operating upon columns within the row) are known in the art as xe2x80x9cpage modexe2x80x9d DRAMs.
In light of the foregoing, in the event that a memory controller has several memory accesses to be done sequentially, then once a page is open it makes sense from an efficiency standpoint to examine pending as well as current memory accesses in order to determine which of those pending memory accesses will be to memory locations that are within a currently open page (that is, the row of the request is the row from which a memory controller is currently reading within a DRAM). In other words, assuming a page X is open, if there are four memory accesses A, B, C, and D, waiting to be performed, and assuming the first access A is to page Z, the second access B is to page X, the third access C is to page Y, and the fourth access D is to page W, it is preferable from a memory efficiency standpoint that the data access (i.e., access B) appropriate to the page that is open (i.e., page X) be made first.
Current memory controllers already xe2x80x9clook aheadxe2x80x9d to see if pending memory accesses are destined for currently open pages. Furthermore, at any given time, typically more than one page of memory is generally open. For example under the Direct RDRAM scheme (expected to be available in the near future), it is expected that up to 8 pages per RDRAM chip will be open simultaneously. Thus, if a system has eight RDRAM chips (a reasonable assumption), it will be possible to have up to 64 pages open simultaneously. Thus, when multiple memory accesses are to be sequentially executed, an efficient strategy which may be employed by the memory controller is that it selects which ones of the memory accesses to be executed are intended for pages which are already open, completes those accesses first, and subsequently proceeds with the memory accesses which will require opening new pages. This greatly increases memory efficiency.
Controlling memory access via the use of xe2x80x9clook aheadxe2x80x9d is undeniably valuable. Furthermore, as the foregoing has shown, the ability of the memory controller to schedule memory access is currently becoming more important as both current and future system memories are likely to be able to provide a very large number of open pages of memory simultaneously. It is therefore apparent that a need exists in the art for a method and system which will increase the ability of memory controllers to intelligently schedule accesses to system memory.
It has been discovered that a method and system can be produced which will increase the ability of memory controllers to intelligently schedule accesses to system memory. The method and system provide a memory controller and a requested memory operation buffer structured so that at least one source attribute of a requested memory operation can be identified. In one instance, the requested memory operation buffer has queues, associated with data buses, which can be utilized to identify source attributes of requested memory operations. Examples of such queues are an Accelerated Graphics Port Interconnect queue associated with an Accelerated Graphics Port interconnect, a system bus queue associated with a system bus, and a Peripheral Component Interconnect bus queue associated with a Peripheral Component Interconnect bus where the queues can be utilized by a memory controller to identify the specific bus from which a requested memory operation originated. In another instance, the queues, associated with data buses, are structured such that one or more further source attributesxe2x80x94such as the identity of the request initiator, the priority of the request, whether the request is speculative, etceteraxe2x80x94of particular queued requested memory operations can be identified. In yet another instance, the requested memory operation buffer is structured such that one or more source attributesxe2x80x94such as the identity of the request initiator, the priority of the request, whether the request is speculative, etceteraxe2x80x94of particular queued requested memory operations can be identified.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.